1. Field of the Invention
The present invention relates to imaging of gamma ray sources using scintillating fibers. In particular, the present invention relates to a device and method for determining and utilizing cross-talk adjusted scintillating fibers.
2. Background
Recent advances in diagnostic imaging, such as magnetic resonance imaging (MRI), computerized tomography (CT), single photon emission computerized tomography (SPECT), and positron emission tomography (PET) have made a significant impact in cardiology, neurology, oncology, and radiology. Although these diagnostic methods employ different techniques and yield different types of anatomic and functional information, this information is often complementary in the diagnostic process.
More particularly, the present invention generally relates to scintillating fibers used in PET and SPECT systems. Generally speaking, both PET and SPECT involve the detection of gamma ray photons. They differ in the source and energy of these photons. In the case of SPECT, the photons are directly generated by radioactive isotopes while in the case of PET, the photons are generated during the annihilation of positrons emitted by the radioactive isotopes. These radiotracers or radio pharmaceuticals typically are synthesized from labeled precursors and are inhaled or injected into the patient.
Typical scanners or tomographs include banks of large numbers of scintillating detectors surrounding the patient and are coupled to computerized data acquisition systems. The images of the temporal and spatial distributions of the inhaled or injected radio pharmaceuticals are reconstructed by using mathematical imagery construction tomography. They can provide unique functional information on blood flow and metabolism not easily obtainable by other technologies. In the case of PET, the short half-lives of the isotopes used, make it more convenient to produce them in an on-site cyclotron or other type of particle accelerator. A more recently developed PET system, as disclosed in U.S. Pat. No. 5,103,098 to Fenyves et al., allows the use of longer-lived isotopes that do not require on-site generation. U.S Pat. No. 5,103,098, issued on Apr. 7, 1992, is hereby incorporated by reference in its entirety.
Plastic scintillating fibers have become widely used in particle physics experiments. Commercial versions of these scintillating fibers are generally produced using polystyrene doped with a primary dye and with a secondary dye. Particles generated outside of the fiber travel through these fibers and deposit energy within the fiber that is converted by the fiber into light that can be detected. Scintillating fibers may be used in medical imaging systems where gamma rays interact with the fibers to generate these particles internally within the fiber.
In general, the process by which the particle energy is partially converted into light by scintillating fibers is the same for particle physics and medical imaging and takes three steps. First, the particle deposits energy into the scintillating fiber as it travels along its track. Second, the primary die absorbs part of this energy and emits it as ultraviolet light photons. Finally, the secondary dye absorbs energy from the ultraviolet light photons and emits it as visible light photons that travel down the fiber to be detected.
The distance over which the primary dye and secondary dye absorb energy is described by a primary and a secondary attenuation length. Attenuation length refers to the distance over which the energy flux intensity is reduced by 1/e and is dependent upon the concentration of dye in the scintillating fiber. The primary attenuation length is usually very small compared to the secondary attenuation length. The critical attenuation length for determining where visible light is generated, therefore, is the attenuation length for the absorption of ultraviolet light by the secondary dye. The smaller the secondary attenuation length, the smaller the volume where the ultraviolet light is absorbed and emitted as visible light. In most applications this volume is made as small as possible to prevent visible light from being generated in fibers adjacent to the one containing the primary event. The primary event is the location of the primary energy deposition of the particle to be detected.
Although a prior system utilizing liquid filled glass tubes recognized that communication between an X layer and a Y layer may be helpful, recent work in scintillating fiber research has been in the area of minimizing the secondary attenuation length to reduce communication between fiber layers. This effort has been fueled by the desire to improve the resolution of systems using scintillating fibers, which is determined by the ability to pin-point the location of the primary event. The accuracy of this measurement depends upon the diameter of the scintillating fibers used in the system (larger diameters reduce communication) and the amount of light produced in neighboring fiber layers (indicative of "cross-talk").
"Cross-talk" occurs when visible light is produced in fibers adjacent to the fiber containing the primary event. Cross-talk has recently been dealt with as creating uncertainty as to the location of the primary event. This uncertainty effectively reduces the resolution of the system. Thus, recent scintillating fiber research has sought to eliminate cross-talk by creating a fiber in which a significant portion of ultraviolet light emitted by the primary dye is absorbed within the same fiber in which it is produced. Creating such a fiber is accomplished by reducing the secondary attenuation length as much as possible.
The secondary attenuation length is a direct function of the concentration of secondary dye in the scintillating fiber. Fiber manufacturers can now produce a scintillating fiber with a secondary attenuation length as low as 10 microns by manipulating the concentration of secondary dye in the fiber. However, as mentioned above, the primary effort in the field has been to reduce the attenuation length to its minimum value in order to remove problematic cross-talk.
One of the challenges of detector systems is to provide the exact position of the energy deposition within the detector. A single fiber can produce two of the three coordinates necessary for position determination, but does not provide its location along the length of the fiber. In particle physics experiments, where the energy of the incoming particle is high, this is not a problem, since the particle will traverse several fibers. One can thus obtain the third position information from layers of multiple fibers which are angularly displaced relative to each other, for example, an XY fiber system. The detection of the particle in two adjacent layers, produces an accurate three dimensional position measurement.
In medical imaging systems, however, the particle energy is often so low that there is very little probability that the particle will travel enough distance to deposit energy into two different fiber layers. The shortened attenuation lengths for the secondary dyes found in commercial fibers today have greatly reduced the possibility of energy deposited by a particle in one fiber producing visible light in an adjacent fiber. This has led to the perception that scintillating fibers would not be useful as SPECT and PET detectors.
What is needed, therefore, is a device or method that would allow and take advantage of visible light produced in adjacent scintillating fiber layers used in a medical imaging system, without adversely affecting the resolution or efficiency of the system.